Close-spaced thermionic converter



June 23, 1964 J. M. HOUSTON 3,138,725

CLOSE-SPACED THERMIONIC CONVERTER Filed Sept. 17, 1958 40 AMBIENT/UR 38 f 1 r H 25 /n van/or: John M Houston,

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United States Patent Oil Free 3,138,725 Patented. June 23, 1964 3,138,725 CLOSE-SPACE!) THERMIONIC CUNVERTER John M. Houston, Schenectady, N .Y., assignor to General Electric Company, a corporation of New York Filed Sept. 17, 1958, Ser. No. 761,648 6 Claims. (Cl. 310-4) This invention relates to a thermionic converter producing an electron current flow between a cathode and an anode in response to heat applied to the cathode and more particularly relates to an improved and simplified apparatus for increasing the current flow between cathode and anode and consequently, also the operating efliciency in such a converter by maintaining a uniform and close spacing between the cathode and anode.

In a thermionic converter wherein an electron current is produced by electron emission from a heated cathode, it is important for good efliciency to facilitate a large current flow between the cathode and anode of the device in relation to the heat energy applied to the cathode. One of the inherent impediments to such a large current flow is the space charge effect within the converter which comprises an aggregation of electrons in transit at a location between the cathode and anode. Such a space charge having a relatively high negative charge produces a repelling or retarding force against electrons newly emitted from the cathode and which also are negatively charged, thereby interfering with the free flow of the emitted electrons. In the case wherein the converter elements are enclosed in a gaseous medium, the collision of electrons emitted from the cathode with molecules of the gas becomes a 7 factor reducing the number of electrons reaching the anode or in other words, the current intensity.

In accordance with a feature of my invention, the space charge and molecular collision effects are minimized by producing and maintaining the spacing between cathode and anode very close. In such a case, the anode is positionable at or near a location where electrons forming a space charge would be positioned, the effect of which is to collect the greater proportion of such electrons to eliminate or greatly minimize the space charge effect. In the case of converters having gaseous electron paths, the very close spacing effected herein, presents a relatively short path to electrons whereby the probability of collision between electrons and gas molecules is greatly lessened.

In achieving a close spacing between a cathode and an anode in accordance with conventional techniques, considerable, precise and time consuming effort is involved in construction, assembly and operation of components. In the operation of a device in accordance with such construction, it is subject to adverse effects such as thermal expansion and warping of surfaces making maintenance of such close spacings diflicult. Each of the surfaces must be made with great precision so that the spacing therebetween can be evenly maintained throughout.

It is accordingly a primary object of my invention to facilitate a uniform, close spacing between anode and cathode of a thermionic converter without the necessity for accurate and precise effort in making and mounting the elements relative to each other as heretofore required.

It is another object of my invention to automatically effect a uniform, close spacing between anode and cathode of a themionic converter, notwithstanding the occurrence of distortion or warping of one of the electrode elements.

It is still another object of my invention to facilitate cooling of an anode of a thermionic converter. Pursuant to the aforementioned objects and in accordance with features of my invention the adjacent electrode surfaces in a thermionic converter are maintained closely and uniformly spaced apart by interposing a powder of insulating, refractory material such as alumina, beryllia, thoria,

boron nitride or cerium sulfide between a cathode electrode and a thin, flexible anode electrode which is gently urged toward the cathode electrode by a fluid pressure eifective throughout the area of the anode on the side thereof removed from the cathode. The particles of the powder are of a very small but uniform size and are distributed over the common area between electrodes whereby the anode under the influence of the fluid pressure is forced into engagement with the particles of powder and the anode conforms to the contour of the cathode. Any irregularities in the cathode surface resulting from imperfect construction or warping or other reason are automatically accommodated by the flexibility of the anode electrode. Such electrode surfaces may be adjacent planar, adjacent concentric cylindrical or of various other shapes and the fluid pressure against the anode may be applied by a suitable coolant performing the additional function of maintaining the anode at a desirably low temperature.

Other and further objects and advantages will appear from a consideration of the following detailed description of the invention considered in conjunction with the drawings in which:

FIG. 1 shows in vertical cross-section one embodiment of a thermionic converter having an enclosed chamber for circulating a coolant against the anode thereof, and

FIG. 2 shows in vertical cross-section another embodiment of a thermionic converter having the anode exposed to ambient space.

Referring now more particularly to FIG. 1 of the drawings for a detailed description of my invention, 10 represents generally a thermionic converter according to .one embodiment thereof which includes a relatively rigid cathode 12 of inverted cup shape having an end surface 14 closing an intermediate portion 16 and a flange 18 extending radially from the other end of portion 16. Cathode 12 is preferably of a refractory metal such as tungsten or tantalum and for ease and convenience in construction is preferably made circular in shape although polygonal or other shapes may be used. Although portions 16 and 18 are shown as being relatively thick for purposes of illustration, the same are preferably relatively thin to minimize the transfer of heat from end surface 14 of the cathode to other parts of the apparatus.

An enclosed chamber 20 is established by cathode 12 and an anode 22 joined together by a rigid refractory insulator 24 bonded at respective ends to the flange 18 and anode 22 near their peripheries. Chamber 20 is preferably free of all gases except a low pressure vapor of an alkali metal such as cesium.

According to a feature of my invention, anode 22 is made of a thin, flexible sheet of metal such as nickel which is deformable by pressure differential across its sides to be brought close to cathode surface 14. Anode 22 may be of the order of a few thousandths of an inch thick, but its thickness and its deformation for illustration are shown exaggerated in the drawings. According to another feature of my invention a close uniform spacing is maintained between cathode surface 14 and anode 22 by particles 25 of a refractory powder randomly interspersed between these surfaces and having a substantially uniform number of particles per unit area throughout the area of the electrodes and thereby determine the length of the discharge path between said anode member and said cathode member. Purely as examples, such particles may be composed of alumina, beryllia, thoria, boron nitride and cerium sulfide, it being understood, however, that it is required that such particles be able to withstand the temperatures encountered without damage. As an example the particles 25 of the powder may be of any substantially uniform size within the range from one ten-thousandth of an inch (.0001") to one-hundredth of an inch (.01") thick. The number of particles necessary for maintaining the spacing between surfaces aggregate, in projected area on either of these surfaces, approximatelyone percentum of the total area of either of these. surfaces, Accordingly, presence of the particles in an insignificant interference with current flow between cathode surface 14 and anode 22. The particles 25 of refractory powder are maintained in position by frictional engagement withthe electrode surfaces and which may be increased by the alkali metal condensed on the electrodes.

The pressure differential across anode 22 is achieved by a circulating coolant 26 under pressure sufiicient to gently urge and deflect the anode against the powder particles 25. Coolant 26, which may be mercury, water or any other suitable liquid or gas, is confined by a cap member 28 bonded near its periphery to anode 22 and annularly recessed at 30. Respective inlet and outlet conduits 32 and 34 for admitting and discharging coolant are provided.

Output leads 36 and '38 are conductively connected, respectively, to cathode and anode of the converter to provide a direct output potential of negative polarity at the anode and positive polarity at the cathode as indicated on the d-ravw'ng.

During operation of the converter, heat is applied in any suitable manner to emitting portion 14 to raise the temperature thereof to a point wherein the same emits a copius quanity of electrons with suflicient energy to travel to the anode. With a tungsten cathode the operating temperature of the same may be in the range of from 1500 Kelvin to 2000 Kelvin. For satisfactory operation, anode 22 is maintained at a desirable temperature aproxirnately 500 Kelvin or more lower than the cathode by the coolant 26 circulated in chamber 30 and the pressure of the coolant is sufiicient to gently and uniformly force anode 22 against the powder particles 25. The presence of cesium vapor lowers the work function at the anode in the manner set forth in the application of Volney C. Wilson, Serial No. 698,552, filed November 25, 1957. Inasmuch as the chamber is filled with low pressure gas, the coolant pressure requirements would likewise be low. Under these conditions and during normal operation, the converter may produce an output potential of approximately 1.8 volts.

In accordance with the embodiment of my invention shown in FIG. 2 wherein parts similar in structure and operation to parts in FIG. 1 are designated by the same numerals of the drawing, a converter 40 similar in structure to converter 10 is provided wherein the cap member 28 is eliminated and circulating ambient air functions as a coolant for anode 22. The ambient air pressure of approximately one atmosphere is suffic'ient to force the anode 22 into uniform contact with the refractory powder particles 24. p

In each of the embodiments of invention, the anode conforms to the contour of the cathode by reason of its flexibility and the pressure forcing it against the refractory powder and the cathode. Accordingly, any imperfections in or distortion or warping of the cathode is readily and automatically accommodated by the anode and under all circumstances the spacing between these elements remains the same.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An apparatus comprising a relatively rigid cathode member, a thin, flexible, anode member and a refractory insulating powder having particles uniformly interspersed between said members, means providing a uniform pressure against said anode member on the side thereof re mote from said particles to maintain the anode member in firm engagement with said powder particles and said powder particles in firm engagement with said cathode member and thereby determine the length of the discharge path between said anode member and said cathode member.

2. An apparatus comprising a cathode member adapted to be heated to a temperature of electron emissivity, a flexible anode member having a surface portion spaced from a surface portion of said cathode member, impervious means between said cathode and anode members establishing a sealed enclosure therebetween, said enclosure being filled with a low pressure vapor of an alkali metal and a fine powder of refractory material having particles uniformly interposed between said surface portions to maintain the spacing therebetween equal to the thickness of said particles and thereby determine the length of the discharge path between said anode member and said cathode member.

3. An apparatus for converting thermal energy to electrical energy comprising a cathode member adapted to be heated to a temperature of electron emissivity, a flexible anode member spaced from said cathode member, impervious means extending between said cathode and anode member to form a sealed enclosure therebetween, a fine powder of refractory material having particles uniformly interposed between said members to maintain the spacing therebetween equal to the thickness of said particles, a second enclosed chamber having said anode member as I one wall thereof for receiving a coolant to be circulated therein, the pressure of said coolant forcing said anode member into uniform firm contact with the particles of said powder and thereby determine the length of the discharge path between said anode member and said cathode member.

4. An apparatus comprising a cathode member hav ing a surface adapted to be heated to a temperature of electron emissivity, an anode member spaced from said surface, impervious means extending between said anode and cathode members to form a sealed chamber "therebetween, an alkali metal vapor in said chamber and a powder of fine particles disposed between said anode and said cathode surface to maintain a uniform spacing therebetween and means forcing said anode member intocontact with said particles and thereby determine the length of the discharge path between said anode member and said cathode member. 5. An apparatus comprising a relatively rigid, planar cathode member, a thin flexible, generally planar anode member and a refractory insulatingpowder having particles uniformly interspersed between said members, means providing a uniform pressure against said anode member on the side thereof remote from said particles to maintain the anode member in firm engagement with said powder particles and said powder particles in firm engagement with said cathode member and thereby determine the length of the discharge path between said anode member and said cathode member.

6. An apparatus comprising aconduotive cathode member adapted to be heated to a temperature of electron emissivity, a conductive anode member spaced from said cathode member, a fine powder of refractory material having particles uniformly interposed between said members to maintain the spacing therebetween equal to the thickness of the particles of said powder, and means for circulating a fluent coolant against said anode member to urge the same into engagement withsaid particles and thereby determine the length of the discharge path between said anode member and said cathode member.

References Cited in the file of this patent UNITED STATES PATENTS 1,887,739 Mott-Smith Nov. 15, 1932 2,231,610 Becker Feb. 11, 1941 2,510,397 Hansell June 6, 1950 2,661,431 Linder Dec. 1, 1953 2,759,112 Caldwell Aug. 14, 1956 2,881,384 Durant Apr. 7, 1959 

1. AN APPARATUS COMPRISING A RELATIVELY RIGID CATHODE MEMBER, A THIN, FLEXIBLE, ANODE MEMBER AND A REFRACTORY INSULATING POWDER HAVING PARTICLES UNIFORMLY INTERSPERSED BETWEEN SAID MEMBERS, MEANS PROVIDING A UNIFORM PRESSURE AGAINST SAID ANODE MEMBER ON THE SIDE THEREOF REMOTE FROM SAID PARTICLES TO MAINTAIN THE ANODE MEMBER IN FIRM ENGAGEMENT WITH SAID POWDER PARTICLES AND SAID POWDER PARTICLES IN FIRM ENGAGEMENT WITH SAID CATHODE MEMBER AND THEREBY DETERMINE THE LENGTH OF THE DISCHARGE PATH BETWEEN SAID ANODE MEMBER AND SAID CATHODE MEMBER. 